1. Technical Field
The present invention relates to a stator core which can be used in an electric motor such as a brushless DC motor and a method of manufacturing thereof.
2. Description of the Related Art
A stator core is generally formed by bending an elongate and straight strip made of thin silicon steel into an annular shape, and by making a lamination with a plurality of the bent strips. In this specification, such straight strip is referred to as “straight core”. Each straight core is made by punching laminated wide and thin silicon steel sheets. The straight core includes a plurality of teeth portions, each of which has a T-shape, and a plurality of core back portions, each of which connects two T-shaped teeth portions next to each other.
To explain a conventional straight core, a partial plan view of an unbent straight core is shown in FIG. 10. The conventional straight core 100 is made by punching laminated silicon steel sheets and includes a plurality of teeth portions 111 and a plurality of core back portions. V-shaped notches 113, which define V-shaped gaps opened to one direction, are formed between the teeth portions 111 and each of the teeth portions 111 are connected with each other via thin portions 117 which are integrally formed with the core back portion so as to have narrower width than the core back portion by the V-shaped notches 113. These thin portions 117 and V-shaped notches 113 form bent portions 112.
The stator core, which is used as a laminated stator core of an electric motor, is manufactured by bending the straight core 100 at the bent portions 112 in a direction so as to close an opening of the V-shaped notches 113. And then connecting both end portions of each of the straight core, the straight cores are formed into annular configuration.
In the conventional straight core 100, as shown in FIG. 11 depicting an enlarged view of section B of FIG. 10, circular hole 114 is provided at the bent portion 112 in a manner that the circular hole 114 is connected to a tip portion of the V-shaped notch 113 (indicated by chain line in FIG. 11) so as to form a series of gaps between each of the teeth portions 111 (in FIG. 11, the tip of the V-shaped notches 113 is positioned on a circumference of the circular hole 114) so as to prevent thin portion 117 of bent portion 112 having narrower width from being cracked to make a uniform flexure deformation during bending process and arc shaped fitting portions 115 and 115′ are also provided with the bent portion 112 so as to prevent bounded teeth portion 111 from displacing with each other.
In the aforementioned straight core 100, a pair of projecting portions 116 and 116′ are formed at the intersecting locations where the circumference of the circular hole 114 intersects a pair of oblique lines of the V-shaped notch 113 in a manner that the projecting portions project to oppose with each other. And a narrower gap part 118 is formed between the projecting portions 116 and 116′. In the production process of the straight core, the punching force of a cutting die for punching out the straight core 100 from laminated silicon steel sheets is partially abated at the projecting portions 116 and 116′. Therefore, the depreciation and repair costs of the cutting dies and the maintenance of manufacturing facilities are increased, because partial abrasion loss of the cutting dies become larger and duration of the cutting dies become shorter. Which makes the production cost of the core increase.
However, if the diameter of the circular hole 114 is enlarged so as to equalize the punching force acting on the laminated silicon steel sheets, cross-sectional area of the bent portions 112, which contributes to forming magnetic circuit between bounded teeth portions 111, becomes small. As the result, magnetic reluctance of the bent portions 112 is increased and magnetic properties thereof are deteriorated. And the diameter of the stator core must necessarily be at least a given size for decreasing the magnetic reluctance of the bent portions 112. This requirement stands in the way of making the electric motor be smaller.
In addition, as shown in FIGS. 12(a) and 12(b), because the tip portions of the projecting portions 116 and 116′ are located at the both sides of the narrower gap parts 118, during the bending process of the straight core 100 the contact point of the tip portions act as a fulcrum of the bent portions 112, as a result a deformation stress is regionally concentrated around the projecting portions 116 and 116′ and the stress concentration deforms the steel sheets in a large way and in a broad area around the bent portion 112. The large deformation increases iron loss of the bent portion 112 and deteriorates the magnetic property thereof.
Further, the accuracy of the configuration of the annular stator core (i.e. a roundness, a concentricity or a coaxality of it) is deteriorated by the deformation of the bent portions 112 or a dispersion of an occurrence of the deformation during the bending process. As a result an increased-noise and a vibration of the electric motor are made.
An object of the present invention is to reduce the production cost of a stator core by increasing the life time of a cutting die for manufacturing stator cores.
Another object of the present invention is to maintain magnetic properties and improve accuracy of work of a stator core by preventing a straight core from deforming during bending process.
And another object of the present invention is to reduce the production cost of an electric motor and to minimize a noise and a vibration during the electric motor operation.